3. EMERSED PLANTS

3.1 Cattail (Typha sp.)

Cattails are marginal plants that grow 180 to 240 centimeters tall.The leaves are long and narrow with sheathed stem. Cattails have thickbranching rootstocks. Thousands of seeds are produced in cylindricalspikes which are brown when mature.

Control : MechanicalRecommendation : Early removal

WESTLY R. ROSARIOWESTLY R. ROSARIO

Para sa karagdagang kaalaman, sumangguni sa:Bureau of Fisheries and Aquatic Resources

NATIONAL INTEGRATED FISHERIES TECHNOLOGY DEVELOPMENT CENTERBonuan-Binloc, Dagupan City, Philippines

Tel. No.: (075) 653-5412; Telefax No.: 653-0385E-mail:bfarniftdc@yahoo.com;westlyrosario@ymail.com

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NATIONAL INTEGRATED FISHERIES TECHNOLOGY DEVELOPMENT CENTERBureau of Fisheries and Aquatic ResourcesDepartment of AgricultureTel. No. (075) 653-5412; Telefax No.: (075) 653-0385E-mail : bfarniftdc@yahoo.com

Published by the ASIAN FISHERIES ACADEMYE-mail : asianfishacademy@yahoo.comTel. No. (075) 653-8851

tilapia responds better with the application of animal manure than withartificial feeds. The nutrients and organic matter content of manureincrease the water-holding capacity of soil, decrease the rate ofevaporation and increase enzymic activity, all of which increase fertilityand crop yield (Schroeder, 1980). Animal manures contain the majorinorganic nutrient components: nitrogen (N), 72 to 79 percent;phosphorus (P), 87 percent; and potassium (K), 82 to 92 percent, inaddition to such trace elements as calcium (Ca), copper (Cu), iron (Fe)and magnesium (Mg) (Taiganides, 1978). Urine which comprises about40 percent by weight of the total daily excreta has higher nitrogen andpotassium levels than feces. Phosphorus comes mainly from feces exceptfrom pigs which have more nitrogen and potassium. Animals fed withroughage ration excrete more potassium than those fed with highconcentrate rations.

The chemical composition of manure also varies depending uponthe animal species, age and condition of animals, nature and amount ofmanure, and the handling and storage of the manure before use.

2.1.1 Manures in the food web

The information on the dynamics of the food web is stillincomplete. Nevertheless, Schroeder (1980) and Woynarovich (1980)stated that manure may enter the food web in several ways:

(a) As food consumed directly by fish

Most laymen think that animal manures are applied in ponds tofeed the fish. This may be true. The response of fish to manuring dependson the method of fertilization used. When manures are applied extensivelyin heavily stocked and not well-fertilized ponds, the fish will consumethe manure as food. This observation prompted scientists to test the useof manure as an ingredient of fish feed pellets. The experiments weredone in tanks and cages to avoid or minimize the presence andcontribution of plankton normally abundant in ponds. The results showedthat the fish fed with the manure-mixed pellets failed to grow to a sizecomparable to those which were fed with conventional pellets. It was

Appendix 1

Some Aquatic Weeds Growing in Fishponds in Central Luzon

1. SUBMERSED PLANTS

1.1 Bushy Pond Weed (Najas sp.)The bushy pond weed is a very common submersed plant. It

resembles Bermuda grass but grows underwater. The leaves arenarrow and ribbon-like and deep green to purplish green in color.

Control : MechanicalPrevention : Shading phytoplankton

2. FLOATING PLANTS

2.1 True Floating Plants2.1.1 Duckweed (Lemna)

The duckweed is a small, floating plant, green in color and aboutone centimeter long across with two to four leaves. Extending beneatheach leaf is a single, short root.

Duckweed floats freely and is moved by wind current.

Control : Mechanical, by net, rake or similar devicePrevention: Avoid introduction

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nutrients, are eaten by pelagic as well as bottom-feeding fish. Spataru(1976) and Collis and Smitherman (1978) showed that bottom feedingtilapia consume detritus as well as plankton in significant quantities. Schroeder(1980) cited three studies which concluded that the microbial community indetritus provided essentially all the nutritional requirements of fish.

2.1.2 Application

Proper manure application is essential for maximum fish yields.According to Schroeder (1980) an even distribution of manures overthe pond area is desirable. Over fertilization may result in the accumulationof manure at the pond bottom. Accumulation of more than a fewmillimeters of manure at any portion of the pond will result in anaerobicdigestion which will produce an interstitial pH of about 6.5. Ammoniaconcentration will increase while microbial production will decrease.High microbial activity and production will only be found within the initialone-or two-millimeter layer of the sediments. Therefore, continuousgradual application of manure should be done to avoid manure overload.

There seems to be a maximum amount of organic matter that apond can digest per unit of time (Schroeder, 1980). Broussard et al.(1982) attained an extremely high production of Tilapia nilotica(SEAFDEC strain) in manured ponds at the Freshwater Fish Hatcheryand Extension Training Center (FFH-ETC), Muñoz, Nueva Ecija. Thiswas due to the use of a fertilization rate of 3,000 to 4,000 kilograms perhectare per month of chicken manure applied weekly. The fertilizationrate mentioned still needs verification. A study of the FFH-ETC usingfive different manuring rates showed that a total production of 35.91kilograms per hectare per day of tilapia can be attained by applying5,000 kilograms of chicken manure per hectare per month. Schroeder(1980) reported that fish yields of 20 to 32.5 kilograms per hectare perday over 120 days were attained with swine manure in polyculturesystems.

2.1.3 Storage

Animal manure can have a greater fertility effect when properlycollected, stored, and used. When exposed to rain, manure will lose

9. HARVESTING

For better regulationsof fish density in ponds,harvesting methods thatefficiently remove most ofthe fish should beemployed. A small numberof fish left in the pond after harvest will probably by caught during thefollowing harvest. However, if they rare too numerous, the growth rate ofthe fish stocked in the following production cycle may be affected.

9.1 Thinning

Partial harvest can begin in the later part of the growing season.Wild spawning normally occurs in this part of the culture period when thefish reach sexual maturity. The population should be thinned out to allowfurther growth of the remaining fish. Either fingerlings or marketable sizefish are partially removed. If thinning is done for marketing only, a netwith a mesh that can catch the desired size for fish should be used. Thiswill allow small fish to escape and grow bigger faster.

The principle of thinning or selective harvesting should not beabused. In tilapia culture thinning is effective only if done once. Totalharvest of tilapia must be done one to two months after.

9.2 Seining

Although seining is often recommended in harvesting fish in thepond, it is not an effective method to insure total harvesting of the stock.Tilapia often burrow themselves into the mud to escape from the net.

9.3 Draining

Although seining, which is the fastest method of harvesting, thepond should be drained so as to eliminate all the predatory species andcompetitors and the fish which may have escaped from the net.

If needed, the pond should be poisoned to insure the totalelimination of any species left in the pond. Draining is necessary as exposureof the pond bottom to sunlight will increase its fertility.

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kilograms per hectare per month in addition to chicken manure which isapplied at 3,000 kilograms per hectare per month. The 16-20-0 gradefertilizer is more readily available than other fertilizers with higherphosphorus content.

3. WATER MANAGEMENT

does not inhibit the growth of T. aurea. It is possible that commoncarp can increase the growth of tilapia. Common carp, when large enough,prey on tilapia fry (Spataru and Hepher, 1977).

8.2.2 Predators in tilapia ponds

The use of predatory fish in tilapia ponds is not widely appliedand is practiced mainly on an experimental scale. Nevertheless, mosttilapia fish-farmers are intrested to know more about this.

Bardach et al. (1972) stated that growing of carnivorous fishsometimes reduces costs of production by taking advantage ofinexpensive sources of food. Wild spawning is a familiar problem inraising tilapia, and introduction of a piscivorous fish like mudfish (dalag),catfish (hito), or goby (biya) may have an immense contribution in solvingte problem, if properly employed. Predators will control wild spawning.However, they should be relatively smaller than the stocked fish, andshould not breed spontaneously in the ponds.

8.3 Monosex Culture

The culture of a single sex tilapia population lengthens the cultureperiod. The fish can be gown to an older age, thus a larger size is attained.IN tilapia, the males are highly preferred. Male tilapia grow two to threetimes faster than the females.

8.3.1 Manual sexing

The sexing of tilapia is done by observing the genital papilla ofthe fish. The male has one orifice while the female has two orifices.Usually, the female has a smaller genital papilla.

It is advisable to sex tilapia at a younger stage. However, lesserror is committed when sexing bigger tilapia of about 35 grams.

Manual sexing is laborious, time-consuming, and may entail greatmortality due to excessive handling. Disregarding females for culturemay not be economical.

Availability of goodquality water is a basic andvery important considerationin culturing fish. The watershould be free of toxic-chemical contamination and of unwantedpredatory or wild fishes, and must be available when needed.Oftentimes, however, such ideal conditions are rare, if not absent, inmost small –scale fishpond operations.

3.1 Source

In Central Luzon, particularly in Nueva Ecija and Bulacan, watersources for freshwater fishponds are deep wells and communal irrigationcanals; in Bataan, free-flowing wells and springs; in Tarlac, rivers, deepwells and rainwater; and, in Zambales, deep wells and streams.

Precautionary measures should be employed when using waterfrom rivers, streams, and communal irrigation systems. A simple biossaycan be done by using tow simple techniques: two or three fish areplaced in a pail full of water from the source to be tested and observedfor at least one-half day; or, a hapa or scoop net wit three to five fishsituated 25 to 50 meters upstream of the water source. The water issafe for use when the fish remain alive after the test period. Ocularinspection or nasal testing to determine the presence of toxic substancesis generally ineffective.

3.2 Depth

The definite water depth for any specific culture unit or speciesof fish is not well defined. Nevertheless, deeper water impedes highfluctuation levels of water temperature. Breeding of tilapia is enhanced

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long as other factors are not limiting, the growth of fish per day will bebetter. However, the growth of fish will be slow at higher density stockingbecause the capacity of the natural food to support the fish populationwill be limited to a certain extent. The maximum physiological growthof tilapia is attained at low stocking density (J. Chervinski, personalcommunication).

Various stocking rates have been reported. Guerrero (1983a)recommends a stocking density of 10,000 to 20,000 tilapia fingerlingsper hectare for plankton ponds. Also, a stocking rate of 20,000 to30,000 tilapia fingerlings is being used by Mr. Magno Velayo of Gapan,Nueva Ecija (Guerrero, 1983b). Sevilleja (1983) reported an averagestocking rate of 12,748 fingerlings per hectare per cropping for tilapiamonoculture in Central Luzon.

The Freshwater Fish Hatchery and Extension Training Centerin Munoz, Nueva Ecija recommends a stocking rate of 20,000 to30,000 tilapia fingerlings per hectare per month in well-fertilized pondswhen using the Center’s strains of tilapia. According to M. Broussard,less fingerlings are produced at a stocking density of 30,000 fingerlingsper hectare.

4.1 Factors that Influence the Selection of Stocking Densities in Ponds

A common mistake in grow-out production of tilapia in pondsis the selection of proper stocking densities. The failure to select themost appropriate stocking density will result in poor growth and lowmarket value of the fish produced. To insure profit, the stocking densitiesin ponds must be selected based on the following factors:

(a) Market demand

Market size preferences vary, depending on the place. In manyrural areas, smaller fish (less than 100 grams) are highly saleable.Nevertheless, in commercial fishpond operations that require middlemenin the marketing of the produce, large fish command higher prices thansmall ones.

with accuracy. In practice, however it is unnecessary and uneconomicalto balance supplemental diet for fishponds according to the absolutenutrient requirements of the fish (National Academy of Science, 1977).

Supplemental feeding may be needed when culturing tilapia inponds. Guerrero (1983) reported that supplemental feeding ofT. nilotica may be needed during the second and/or third months ofculture if plankton growth decreased and growth of fish is slow.

8. CULTURE SYSTEMS

8.1 Monoculture

The very common and widely adopted culture practice in theproduction of food fish in the country is monoculture. Monoculture isthe rearing of a single species of fish in a pond (Stickney, 1979).

(a) Yield

A survey conducted by Sevilleja (1983) in Central Luzonindicated an average annual fish yield of 1,011 kilograms per hectarewhich is still below the projected target yield of four to six tons perhectare per year by Guerrero (1983a).

(b) Problems

Tilapia fish-farmers are commonly beset with basic technicalproblems regarding the use of the right amount and method of fertilization,use of appropriate stocking densities, control of aquatic weeds andpredators, water management, and harvesting techniques which havebeen discussed.

It must be pointed out that the most economical stocking rate isnot necessarily that which results in the highest growth rate per fish perday, but rather that which results in the highest yield per unit area.Performance of the selected tilapia strains should also be considered.Some strains of T. nilotica were found to be inferior. The inferior strainsdo not respond well to fertilization, reproduce prolifically and cease togrow earlier (M. Broussard, personal communication).

In many tilapia ponds, wild spawning is very common.Indiscriminate spawning may reduce fish yields to uneconomical levels.

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widely used manure by fish-farmers. Its use insures much higher yieldthan when cow or carabao manure is used. The capacity of the pond toaccommodate fish can be further escalated when inorganic fertilizer like16-20-0 is added with the manure into the pond.

Fertilizers are more effective in supporting higher fish populationwhen they are applied in short and regular intervals, i.e., daily or weeklyapplication. (For further discussion on pond fertilization, please referto Section 2, Pond Fertilization.)

(e) Fish-farmer’s resources

Low-density stocking may be recommended for the fish-farmersbelonging to the low-income group. The high production inputs requiredin higher stocking density may prohibit its adoption by the farmer. Therecommended stocking ratio must conform with the ability of the fish-farmer to supply the needed inputs.

5. AQUATIC WEEDS

Generally, aquatic weeds are abundant in freshwater ponds.However, a well-managed culture pond should not have weeds or floatingaquatic plants at all.

5.1 Adverse Effects of Aquatic Weeds

(a) Stickney (1979) stated that aquatic plants impede solar exposureof the pond water. The productivity of the pond is influenced byphotosynthesis. Light must penetrate sufficiently into the water topromote photosynthetic activity.

(b) Aquatic weeds utilize soil nutrients that might otherwise support thegrowth of phytoplankton. An experiment at Auburn Universityshowed how water hyacinths remove nutrients in the water andreduce phytoplankton production.

(c) Growing weeds hinder effective grazing of stocked fish and exposethem to predation by other aquatic animals. Another problem resulting

weed growth. Ponds are so designed that sufficient water is available allyear round to maintain water levels to the maximum.

(b)Pond fertilization

The effect of pond fertilization on weed control is indirect.Application of fertilizer induces and maintains the growth of phytoplanktonthat shade pond bottom. Hence, the growth of submerged weeds isprevented.

6. PREDATORS AND OTHER PESTS

Fish production in ponds is commonly affected by some pestsand predators. Predators are organisms which prey on the fish beingcultured and the animals that complete for food or space are calledcompetitors.

6.1 Birds

Herons, kingfishers and other birds should be discouraged fromfrequenting the ponds. They devour fish and fingerlings. Birds are alsocarriers of parasites. They are kept away by shooting or trapping. Pondsconstructed without shallow areas are not attractive to birds.

6.2 Snakes

Snakes prey on small fish. Nevertheless, there are but a few toworry about. Banks and dikes must be kept clean to prevent snakesfrom harboring in the ponds.

6.3 Frogs

Fry and fingerlings are eaten by frogs. Tadpoles also competewith the fish for space. Frogs are seldom found in well-fertilized and wellstocked ponds. Their presence can be controlled by removing their eggsacks from the pond water.

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